A Method to Design Lumped-element Tunable Bandpass Filters with Constant Absolute Bandwidth

A simple and efficient design method for tunable filters on lumped elements is presented. The method allows designing bandpass filters with tunable central frequency and constant absolute bandwidth. The filter circuit consisting of series LC-tanks and impedance inverters is used in contrast to the conventional approach. Theoretical restrictions of the method as well as practical factors limiting its application are discussed. Feasibility of the method is demonstrated by a design example of the three-pole constant-bandwidth tunable filter using varactor diodes as controlling components. A good agreement between simulated and measured frequency responses of the tunable filter is shown.